Rotary screw apparatus



July 14, 1942. A. LYHoLM Er'AL ROTARY SCREW APPARATUS Filed Feb. 28,. 1939 III Patented July 14, 1942,`

OFFICE ROTARY seREwAPPARA'rUs Alf Lysholm and Hans Robert Nilsson, Stockholm, Sweden', assignors, by mesne assignments, to Jarvis C. Marble, Leslie M. Merrill, and Percy H. Batten, as trustees y Application February 28, 1939, Serial No. 258,866 y In Sweden March l, 1938` s claims The present invention relates to rotary screw apparatus applicable for use either as a pump or a compressor, or as a motor, depending upon its direction of rotation and has particular reference to rotary screw machines of the kind inwh'ich the cooperating rotors are formed with threads or lobes of the `double spiral type and commonly referred to as the herringbone type. Still more particularly, the invention relates to apparatus of the above kind in which cooperating rotors of unlike` form are employed havingf intermeshing surfaces which on one rotor are generally convex in section and on the cooperating rotor are generally 'concave in section. In this type the pitch circles of cooperating rotors are such that the convexthreads `or lobes. of oneof two lcooperating rotors lie entirely or substantially en'- tir-ely outside of the pitch circle of -the rotor, this rotor being conveniently referredV to as thev male rotor and the cooperating rotor referred to as the female rotor. l'

The invention further refers particularly to apparatus of the kindunder consideration which is adapted to be operated at high speed with very small clearance between the intermeshing surfaces, such clearance being considered as space packing rather than contact packing, since with space packing high peripheral speeds may be employed because of .the absence of friction between relatively moving surfaces and moreover, if

desired, the apparatus may be run dry for handling gaseous media. w

For apparatus of the character under yconsideration, it is highly important fromthe standpoint of emciency of operation that the line of minimum clearance, which for convenience herein may be considered as the line of contact, be-

tween the intermeshing surfaces of the rotors be maintained as short as possible, since these lines of contact form boundaries dening in part the working spaces of the apparatus, andthe less characteristics provided by this kind of structure.

The nature of the disadvantage which the present invention eliminates, and the manner in which this disadvantage is overcome by application of the principles of this invention, may best be understood from a consideration of the following description of apparatus in which the invention is embodied in conjunction with the use of a preferred form`of thread profile of the kind forming the subject matter of the co-pending application of Alf Lysholm Serial No. 183,664, filed January 6, 1938 (now Patent No. 2,174,522, dated October 3, 1939), although it is to be understood that the present invention is not `limited in its broader aspects to use with this specific kind of thread profile, but may find application with other specic profiles having the same general characteristics.

4 In the accompanying drawing,-in which suchv apparatus is illustrated:

Fig. 1 is a centrallongitudinal section vthrough apparatus embodying the invention, which is designed to be employed as a fluid pump;

' Fig. 2 is a` section of the casing structure taken ance space produced in the apparatus by rotor `structure of the type under consideration.

Referring now to the drawing, the apparatus comprises .a casing I0 in which vare rotatably mounted two intermeshing rotors I2 andl I4, of

which only the vrotor I2 is shown in Fig. l. Rotor I2 is provided with convex threads or lobes I6 and I8 meeting at alrapexr midway of the length of the rotor to yform a.lobe or thread of herringbone form. The rotor I4, as will be observed from Fig.v 3, is provided with concave grooves with which' the lobes .of rotor I2 c`ol 'operate in intermeshing relation. The grooves '20, like the lobes on rotor I2, are of double As win be observed from Fig. 3; the lobes la' of rotor I2, of which there are two in the embodiment illustrated, are formed with dissimilarly curved convex flanks, the profiles of which are indicated by lines I8a and IBb in Fig. 3.

The grooves of the rotor I4 have dissimilarly curved concave surfaces 20a and 20h.

,2 L Surfaces I8a and 20a are, in the embodiment illustrated, of arcuate form in cross-section and of substantially the same radius. Surfaces I8b substantially coincides with the pitch 'circle of rotor I2 which may for convenience be considered as.the male rotor, while the outer cylindrical surface of rotor I4, which for convenience may be considered as the female rotor, coincides with the pitch circle of this rotor.

In the embodiment illustrated, the apparatus formed 'by the edges 'of the groove in substann tially contacting relation with the surface of the ank 18a., A similar condition will obviously exy ist to the other side of the central transverse or provides a displacement pump capable of' han-v dling incompressible fluid media, the fluid entering through the inlet port 28 and being dis- -charged through the outlet port 30 in the direction indicated by the arrows of Figs. `1 and 2.

In the operation of the'apparatus, working spaces are formed by the spaces between adjacent rotor lobes and the casing, and as the rotors are turned in a direction to cause the apexes of the convex lobes to enter the apexes of the cooperating grooves, the volumes of the working spaces are progressively decreased.

By suitably choosing the areas of the inlet and outlet ports, the working spaces may be opened up to communication with the outlet port-before the volume of the; closed working space is diminished after passing out of communication with the inlet port, sothat then the apparatus may operate as a displacement pump.

By diminishingthe area of the outlet port so that threads progressively enter cooperating grooves after a given work space has passed out of communication with the inlet port, and before versely, if the direction of rotation' of the apparatus is reversed from that for which it is uti-- lized as a pump or compressor as the case may be, the 'apparatus may be operated as a motor which may be of either displacement or expansible chamber type. In such event the port 30 would 'become the inlet port and the port 28 the outlet port `so that in so far as the present invention is concerned, these ports may have either of these functions.

Let it be assumed, for the purpose of explanation, that the apparatus is to be used as a pump or compressor as indicated in the drawing.

In such case the herringbone lobes, entering their'cooperating grooves, displace fluid from the latter progressively toward the apex plane of the -herringbone at the center of the apparatus.

However, before the apex of any given lobe enters completely into the corresponding groove, a position is arrived at as illustrated in Fig. 3, which from the figure will be observed results in the formation of a clearance space 36 between the flank I8a of rotor l2 andthe sides 20a and 20h of rotor i4, which space is lof generally crescent-shaped form 'in cross-section.

Because of the contact line between the surfaces f the intermeshing lobe and groove, this space 36 assumes the form to one side of the central transverse plane of the apparatus. like that shown in perspective in Fig. 5. The cross'- sectional area of this space diminishes to secapex plane so that the shape of the clearance space formed when the rotors are in the position of relative rotation shown in Fig. 3 will be generally V-shaped, with each arm of the V having the general shape of the space shown in Fig. 5.

With the rotors moving in the direction of ar- -rows 23 and 34 in Fig. 3, it will be apparent that the volume of the' clearance space which is trapped between the flanks of the lobes I6 and I8 and the walls of the cooperating groove, will be decreased to' anxultimate volume of substantially Zero 'by the time the rotors have moved fromthe position shown in Fig. 3 to a position wherein the profile of the dank 18a at the apex coincides with the side 20a of the groove at its apex.

This condition would of course produce destructively high pressure of any fluid trapped in this clearance space, even thoughsuch fluid were of compressible nature, if the desired conditions of operation were to be employed, which conditions are high speed of rotor operation and minimum practical clearance or space packing between the rotors. Obviously, under such conditions, even With a compressible fluid, the fluid could not escape through the space packing at sufficiently rapid rate to avoid building up pres? sures of destructively high value.

In order to avoid this condition, a relief charnber is provided by recessing the forward face of the male rotor' lobe or the forward face of the female rotor groove, or both, at the apex zone of the rotors, that is, the area immediately adjacent to the apex plane and preferably on both sides of that plane.

When the rotors are in the position shown in Fig. 3 communication between the clearance space and the discharge side of the rotors has been cut 01T by the entry of the summit l8c of the male lobe into the cooperating groove. As the rotors revolve beyond thiscut off position the volume of the clearance space is progressively decreased to its theoretical zero value, which in the case of the prof-lle illustrated occurs when the point on the summit I8c lying at the apex of the lobe moves to a position in which it is in the plane coinciding with and joining the parallel axes of the rotors. When the rotors are in this position the lobe surface |8a and the vgroove surface 20a have moved into substantial contact with each other at' the apex plane and the clearance space volume has been reduced to substantially zero. If thefapparatus is operating to pump or compress a gaseous medium, the gas trapped in the Aclearance space can flow into the relief chamber 38 as the volume of the clearance space diminishes, even if the relief chamber is not in communication with the inlet side of the rotors so as to act as a by pass. This however is not desirable, since if the volume of gas in the clearance chamber is forced into a closed relie-f chamber in addition to the gas already there, it results in compression of thel gas in the relief chamber to relatively high pressure, which pressure is dissipated when the movement of the rotors brings the relief chamber into communication with the inlet side of the rotors. Such compression and subsequent expansion represents uselessl work and a loss and is therefore to be avoided.

Preferably the construction is as shown in the drawing,whereby the relief chamber is adapted to come into communication with the inlet side l of the rotorswhen (but only when) the clearance space is closed, so that as the clearance space is thereafter decreased in volume the fluid in it can pass to the inlet side -of the rotors. From inspection of Fig. 3 it will be seen, how this can be accomplished with the relief recess shaped and located as shown in Figs. 1 and 3. In Fig. 3 the clearance space has beenclosed, and while in the plane of this figure (which is the apex plane) the chamber 38 is not yet in communica'- tion with the inlet side, it must be remembered that the arm of chamber 38 below the plane of the paper curves upwardly as viewed in Fig. 3.

This will became apparent by Ireference to the shape of the .relief chamber as shown in Fig. 1. Furthermore the outer edge of the groove surface a curves downwardly in Fig. 3 below the plane of the paper, so that somewherel below this plane the edge of the groove crosses the relief chamber to provide an opening establishing communication between the relief chamber and the inlet side of the rotors.

tained when the rotors are inthe position shown in Fig. 3.

It will be apparent from the foregoing that the present invention is readily applicable tonumerous different specific forms of apparatus of the general type under consideration, whether such apparatus be used as pump, compressor, or motor, and it is accordingly to be understoodvworking spaces for fluid, the surfaces of each of said lobes and its cooperating groove forming The exact position of rotation of the rotors at which the groove edgecrosses the relief chamber may vary, but if the apparatus is to be used to .pump liquids, this crossing must be established coincidentally or substantially coincidentally with the closing of the clearance space, or in other words, Just when'the rotors reachthe position shown in Fig. 3. This is necessary in order to vent to the inlet side the incompressible contents trapped in the clearance space when the latter is closed and thereafter begins to decrease in volume. For reasons previously explained the relief chamber can vent to the inlet side at a. later time in the cycle if the medium inthe clears ance space is of a compressible nature, but it will be apparent that regardless of the nature of the fluid the relief chamber should not vent to the inlet side before the rotors reach the position of Fig. 3, since if that were permitted.. there would.

be an open path for the blowthrough of anindeterminate amount of fluid from the discharge to the inlet side before the rotors reached the cutof! position of Fig. 3. i

The nature of the relief chamber is thus such that there Ais returned to the inlet sidev only the relatively very small quantity of fluid trapped in comparison with the `total quantity handled by the apparatus, is negligible.

It will be evident that the same result attained by the use of the groove in the flank of the convex lobe may be obtained by making the relief chamber in the form of a recess in the face of [the concave groove. Such an arrangement is i1- lustrated in Fig. 4 where the relief chamber isv formed by the groove 40 which takes the same general wedgeshaped character as the groove 38, as shown in Fig. 1. It will further be evident that the relief chamber may be formed partially in each 'of the rotor structures to attain the same result.

The relief chamber is also useful in case the apparatus is used as a motor in order to avoid the production of undesirably high vacuum as the clearance spaces 36 are progressively formed by reverse rotation of the rotors and increase from substantially zero volume to the volume at- `the closed clearance space, and this quantity, in v a clearance space extending longitudinally of the lobe and groove for a limiteddistance on either lside of the apex plane of the herringbone, said clearance space being closed by the entry of the summit of the lobe into the cooperating groove Vadjacent to said apex plane and being reduced in volume to a theoretical zero value as the rotors turn thereafter to bring the lobe fully into the groove at said apex plane, and recess rmeans formed in a wall defining a part of said clearance space to provide a relief chamber located at the apex zone and in Comunication with the clearance space, said recess means being brought into communication with the linlet side of the rotors by rotation of the rotors but only after said clearance space has been closed by the entry of the lobe summit into. the groove, whereby to prevent undue increase in pressure of the fluid trappedY .in said clearance space as the latter is reduced to substantially zero volume and to prevent blowthrough from the discharge side to the inletV side of the rotors due to the fact that said relief chamber -is brought into communication with said inlet side only after communication between said clearance space and said disch-arge side has been cut oil by the entry of the summit of the lobe. -into the groove as aforesaid. 2. Apparatus according Atofclaim 1 in which the recess is of generally wedge-shaped form having arms extending axially -to each side of the apex ofthe lobe and having maximum crosssectional area at its axially central part.

3. Apparatus according to claim 1 in which the recess is located in the apex zone of the forwardly facing convex flank of the male rotor which constitutes one of the walls of said clearance space.

4. Apparatus according to claim 1 in which therecessis formed in the concave surface of the apex zone of the forward side of the groove l I which forms a part of the wall of said clearance space.

5. A rotor for use in rotary screw apparatus, said rotor having a plurality of herringbone lobes, said lbes having convexly curved flanks, and each of said lobes being provided with a recess in the forwardly facing one of its flanks at the apex zone of the rotor, said recess being located between the root and summit circles of the lobe and having arms extending to each side of the apex plane of the lobe, said arms extendingonly a minor portion of the distance from said apex plane to the respective ends of the rotor.

6. A rotor for use in rotary screw apparatus, said rotor having a plurality of herringbone lobes, said lobes having convexly curved ilanks and each of said lobes being provided with a recess vin the forwardly facing one of its anks at the apex zone yof the rotor, saidvrecess being located betweenthe root and summit circles of the lobes;

andthe' cross-section of said recess being great- 10 estfat the apex plane of the rotor and diminishing axially in both directions Aaway from said plane. y

'1. A rotor for use in rotary screw apparatus,

said rotor having a pluralityof herringbone 15 groovesi.- said grooves having concavely curved` sides, andl `each of said groovesbeing provided v with a. recess formed in the forward one of said -sides between the root and top circles of the groove and` having arms extending to each side of the apex plane of the groove, said arms ex tending only a minor portion of the distance from said apex plane tc the respective ends of the rotor.

8. A rotor for use in rotary screw apparatus, said rotor having a yplurality of herringbone grooves, said grooves having concavely curved sides and each of said grooves being provided with a recess' formed in the forward one of said sides between the:root and top circles of the groove, and the cross-section of said recess being greatest at the apex plane of the rotor and diminishing axially in both directions away from said plane.` l ALF LYSHOLM.

HANS ROBERT NILSSQN. 

